In the yeast Saccharomyces cerevisiae, the biosynthesis of inositol- containing sphingolipids is essential for the growth and viability of this organism. Little information is known regarding the function of these lipids in yeast or how the yeast cell regulates the biosynthesis of these essential membrane components. An important step in the biosynthesis of the yeast sphingolipids is the transfer of phosphoinositol from phosphatidylinositol to ceramide forming inositolphosphoryl ceramide, one of the major yeast sphingolipids and precursor to other yeast inositol- containing sphingolipids. This reaction is catalyzed by the enzyme phosphatidylinositol:ceramide phosphoinositol transferase (IPC synthase). A significant biological role for this enzyme is probable since it catalyzes an essential step in the synthesis of the yeast inositol- containing sphingolipids. Furthermore, this enzyme is found at a critical branch point in the yeast phospholipid biosynthetic pathway and regulation of this enzyme is likely to limit the synthesis of the yeast inositol- containing sphingolipids. The major goal of this research is to gain a better understanding of the regulation of IPC synthase and its role in the synthesis of the yeast inositol-containing sphingolipids. As a first step, a procedure for the purification of the yeast IPC synthase will be developed. A detailed enzymological study of the purified enzyme will then he undertaken in order to gain a better understanding of the factors that regulate the activity of this enzyme. Antibody probes will be raised against the pure enzyme and used to investigate the regulation of IPC synthase expression by inositol and the growth phase. These studies should provide important information regarding the regulation of IPC synthase. Lastly, mutants defective in IPC synthase activity will be isolated and yeast sec mutants defective in protein secretion will be screened for defects in IPC synthase activity. Characterization of yeast mutants defective in IPC synthase activity should provide new information which may lead to the elucidation of the physiological function of the yeast inositol-containing sphingolipids. Since there is now a considerable amount of effort being focused on the role of phospholipid and sphingolipid breakdown products and metabolism in controlling cellular proliferation and membrane signal transduction, it would appear that further investigation of the function of the yeast inositol containing sphingolipids is warranted. Furthermore, a better understanding of sphingolipid metabolism in yeast should shed light on the ultimate function of sphingolipids in the membranes of other eucaryotic organisms.